The present invention relates to servo systems in disc drives. In particular, the present invention relates to compensation for errors in servo systems.
Disc drives read and write information along concentric tracks formed on discs. To locate a particular track on a disc, disc drives typically use embedded servo fields on the disc. These embedded fields are utilized by a servo subsystem to position a head over a particular track. The servo fields are written onto the disc when the disc drive is manufactured and are thereafter simply read by the disc drive to determine position.
Ideally, a head following the center of a track moves along a perfectly circular path around the disc. However, two types of errors prevent heads from following this ideal path. The first type of error is a written-in error that arises during the creation of the servo fields. Written-in errors occur because the write head used to produce the servo fields does not always follow a perfectly circular path due to unpredictable pressure effects on the write head from the aerodynamics of its flight over the disc, and from vibrations in the gimbal used to support the head. Because of these written-in errors, a head that perfectly tracks the path followed by the servo write head will not follow a circular path.
The second type of error that prevents circular paths is known as a track following error. Track following errors arise as a head attempts to follow the path defined by the servo fields. The track following errors can be caused by the same aerodynamic and vibrational effects that create written-in errors. In addition, track following errors can arise because the servo system is unable to respond fast enough to high frequency changes in the path defined by the servo fields.
Written-in errors are often referred to as repeatable run-out errors because they cause the same errors each time the head passes along a track. As track densities increase, these repeatable run-out errors begin to limit the track pitch. Specifically, variations between the ideal track path and the actual track path created by the servo fields can result in a track interfering with or squeezing an adjacent track. This is especially acute when a first written-in error causes a head to be outside of an inner track""s ideal circular path and a second written-in error causes the head to be inside of an outer track""s ideal circular path. To avoid limitations on the track pitch, systems that compensate for repeatable run-out errors are employed.
One existing technique for repeatable run-out error compensation involves storing time-domain compensation values in the form of a compensation table on discs in the disc drive. These compensation values are injected into the servo loop to compensate for repeatable run-out errors. Typically, a time-domain compensation value for each servo sector is required to be stored in the compensation table. The resulting large compensation table needs to be stored on discs in the drive due to the large memory requirement.
The present invention addresses these and other problems, and offers other advantages over the prior art.
The present system relates to a repeatable run-out error compensation scheme that employs a compression technique that is integral to the compensation algorithm and stores frequency-domain compensation values in the compensation table, thereby addressing the above-mentioned problems.
A method and apparatus for compensating for repeatable run-out errors in a disc drive is provided in which transfer function values for a servo loop in the disc drive are first determined. A sequence of repeatable run-out values for a portion of the disc drive is then determined. A transform is applied to the sequence of repeatable run-out values to obtain frequency-domain repeatable run-out values. Each frequency-domain repeatable run-out value is divided by a respective transfer function value to produce a sequence of frequency-domain compensation values which are then stored. An inverse transform is applied to the frequency-domain compensation values to obtain a sequence of time-domain compensation values. The sequence of time-domain compensation values is inserted into the servo loop.
These and various other features as well as advantages which characterize the present invention will be apparent upon reading of the following detailed description and review of the associated drawings.